1. Field of the Invention
The present invention relates to the field of use of digital image capturing. More particularly, the present invention relates to image sensors with built-in analog-to-digital converters.
2. Description of Related Art
Typically, imaging sensor arrays have separate analog-to-digital (A/D) circuits which are located off of the array to digitize the captured pixels. The array is composed of a grid of pixel sensors, each pixel sensor generating a charge when the pixel sensor is exposed to a light source to which it is directed.
During operation, the array first captures the image by having each pixel sensor generating and storing an amount of charge corresponding to the amount of light it is exposed to. Then, the array sends the charge stored in each pixel sensor to the A/D circuit over an analog transfer circuit. The A/D circuit then produces a digital representation of the amount of charge received from the pixel sensor.
As the A/D circuit is often located some distance away from the array, the length of the analog lines which are used to send the analog pixel data between the array and the A/D circuit are quite long. As the length of the analog lines is increased, noise and power effects must be taken into consideration to avoid the loss or degradation of the captured signals.
Due to noise considerations, the number of analog lines used to transfer pixel signals from the array to the A/D circuit may be limited to avoid cross-talk or any of the other problems which are caused by interference between these signal lines. Additionally, if multiple independent A/D circuits are used, mismatches between them can cause undesirable artifacts in the resulting image. Therefore, the pixels in an array are typically digitized by transferring each pixel""s charge sequentially to a single A/D circuit. In color sensors, where the array may be considered to consist of several sub-arrays each corresponding to a particular color channel, each color channel may have a single A/D circuit.
It would be desirable to eliminate the difficulties involved in accurately transferring the accumulated charge in each pixel to the A/D circuit. It would also sometimes be desirable to perform some amount of highly-parallel digital processing on the captured image before outputting it from the sensorxe2x80x94also known as digital focal-plane processing. It would be desirable to do both of these things correctly when the scene is illuminated by a photographic strobe, as is usually the case for indoor still photography.
Photographic strobes use a brief arc of electric current to provide a very intense burst of light that lasts for a very brief period of time. The intensity of the burst is not uniform over its duration. Photographic strobes are compact, light-weight and efficient, and allow accurate capture of the color of the object being illuminated.
One method of accomplishing accurate transfers and parallel pixel on-sensor processing would be to perform the analog to digital conversion within the circuitry of each pixel. However, to accomplish A/D conversion within the circuit of each pixel would require each pixel to have its own A/D circuit and building a conventional A/D circuit within each pixel is expensive.
One alternative approach is to implement a very simple A/D circuit that output only one or more bits. By using a technique known as over-sampling, a circuit can approximate a higher precision conversion under certain conditions. However, these conditions are not met when exposure times are effectively very short, for example, only a few multiples of the A/D conversion period, such as when a photographic strobe is used.
The techniques described above measures the charge level on the pixels after they have been exposed for a given period of time, and typically this measurement is done after the exposure has ended. Another approach is to measure, during the exposure, how long each pixel takes to reach a particular level of charge. In this approach, an analog circuit in each pixel constantly compares the charge level to a reference, and when the charge level reaches the reference point, a signal is generated which latches the current exposure time into a register associated with the pixel. This works well if the level of illumination if constant over the image capture interval, and if the image capture interval is many hundreds of clock cycles. However, when photographic strobes are used, these criteria for proper operation are not met.
A system that works with photographic strobes involves having an apparatus having a photo sensor; a comparator coupled to the photo sensor; a switching element coupled to the photo sensor; an AND-gate coupled to the comparator; and, a memory coupled to the AND-gate. In this system, a method is used which has the steps of accumulating charge in a photo sensor to arrive at a photo sensor voltage; incrementing a value in a counter; generating a reference voltage based on the value in the counter; determining whether the photo sensor voltage is greater than the reference voltage; and, if the photo sensor voltage is greater than the reference voltage, then loading a memory with the value in the counter.